1. Field of the Invention
The present invention relates to ring oscillator circuits for generating variable frequencies. More particularly, the present invention relates to ring oscillator circuits for test chips used for evaluation purposes wherein the oscillator frequency may be varied.
2. Description of Related Art
Oscillators are well known in the art and important in providing clock signals in digital logic circuits. Clock generation for semiconductor devices can take many forms including ring oscillators, crystal controlled oscillators, external clock devices, Phase Locked Loops (PLL) on a semiconductor device, Delay Locked Loops (DLL) on a semiconductor device, and various combinations of the above. Crystal controlled oscillators are generally useful for precisely creating a desired frequency, but cannot directly produce the very high frequencies required in some high performance circuitry. Similarly, external clock generators vary greatly in precision and frequency, but they are generally designed to maintain a precise fixed frequency and create global clocks for distribution within a system. As a result, external clock generators tend to be expensive.
Generally, expensive clock generators generate high frequency clocks on a system board and maintaining a clean clock signal at high frequencies is problematic. To overcome this problem, many semiconductor devices use PLL's, which create internal clocks at higher frequencies generally multiples of a lower frequency external reference clock. To be accurate, yet flexible enough to generate a large variety of frequencies, PLL's can be difficult to design. PLL's generally require analog circuit design techniques, and may still not provide the flexibility required for a test chip where varying the frequency of the clock is valuable in analyzing various performance parameters of a test chip.
DLL's may also be used to create clock multiples for an internal clock signal from a lower frequency clock reference. Some DLL's do not require analog circuitry but generally have the same problems of design complexity and lack of flexibility as a PLL solution. However, DLLs are also often used to create phase shifts in an internal clock on a semiconductor device relative to a reference clock. When used as a phase-shifting device, DLL's may be quite useful.